Championship Series. Professional sports leagues commonly end their seasons with a championship series between two teams. The series ends when one team has won four games and so must last at least four games and at most seven games. How many different sequences of game winners are there in which the series ends in

a. 4 games?

b. 5 games?

c. 6 games?

d. 7 games?

e. Assuming that the two teams are evenly matched, determine the probability of each of the outcomes in parts (a)–(d).